The present invention relates to a method for the detection of toxic substances in the waste water supplied to a biological treatment plant.
In such plants where waste waters are biologically treated, the effluents are brought into contact with micro-organisms which metabolize the dissolved organic materials in the presence of oxygen.
The activity of these micro-organisms may be partly or completely inhibited, when the waste waters contain a concentration of toxic materials such that the purifying biomass becomes poisoned, thus reducing or stopping the purifying process.
When these toxic materials are not detected soon enough, the entire biomass of the plant may become poisoned and it then takes several weeks before the purifying plant can resume its activity. The advantage of detecting the presence of toxic substances in waste waters before they can affect the purifying plant, is therefore imperative.
Many methods of detecting toxic substances are already known and used, such as for example methods based on respirometric measurements: wherein toxic substances modify the metabolism of the micro-organisms with which they come into contact, and the variation of metabolic activity can be detected with great sensitivity by studying one essential function which is, respiration.
For example, according to the method described in French Patent Application No. 2,428,842, the liquid for testing is oxygen-saturated and introduced into a reactor where it is mixed with a fraction of bacterial culture and with a nutrient. The culture is so prepared as to have a bacterial aqueous suspension with stable characteristics. The liquid is introduced at the inlet to a reactor formed by a thin long tube and the oxygen content is measured at the reactor outlet, situated at the other end of the tube; the peaks detected in the recordings of the oxygen content measured value indicate the presence of toxic substances. A similar method is described in French Pat. No. 2 266 885 consisting in oxygen-saturating part of the effluent to be tested and causing it to flow through a bacterial filter, at the outlet of which the dissolved oxygen is measured. Where the consumption of oxygen is too low a toxicity alarm installation is activated. Seeding of the reactor is periodically renewed and dilution of the effluent with a nutrient solution becomes necessary when effluent load variations are too great. With such methods, the micro-organisms used in the reactor can react differently from the micro-organisms present in the purifying plant. Moreover, the dilution of the effluent with a nutrient solution entails a dilution of the toxic substances which delays its detection.
In the method according to U.S. Pat. No. 4,260,490, the respiration measured on an activated sludge sample taken from the treatment plant is compared with the respiration measured on an activated sludge sample from the plant having the effluent to be analyzed. If the oxygen consumption speed difference between the samples is below a certain value, determined with a non-toxic effluent used as reference, the monitored effluent is deemed toxic. If on the contrary, said difference is higher than the value determined with an effluent of average oxygen biological intake, the oxygen intake is greater. A computer works out the results of the respirometric tests and if necessary releases a signal in the case of toxicity. This particular method is therefore a discontinuous method which, in addition to computer means, requires two oxygen sensing means as well as two analyzers.
According to yet another method of the type described in French Pat. Nos. 1,567,181, 1,567,182 and 2,067,184, a reactor is used, which reactor receives a certain quantity of activated sludge from the plant oxygenated by injection of compressed air. When these sludge samples reach a constant respiration, a sample of effluent to be analyzed is injected. The oxygen intake from that particular moment is continuously measured and a computer gives the metabolizing curve. Then, if during subsequent measurements the respiration falls below a preset value, the presence of toxic substances is highly probable. This is therefore a discontinuous monitoring method.
Finally, European patent application EP No. 0,009,580 discloses a system permitting control of a biological purifying installation using a measuring device which can receive a mixture or suspension of waste water and recycled sludge collected before its introduction into the basin of the installation. The measuring device comprises a reactor, into which the suspension is introduced and oxygen-saturated, and a measuring cell set apart from the reactor and designed to measure oxygen consumption. The distance between the reactor and the measuring cell is so defined as to correspond to a pre-set reaction period. If the oxygen consumption measurement goes beyond or below first pre-set maximum and minimum thresholds, the flow of recycled sludges to the purifying plant is adjusted. If a second minimum threshold, lower than the first is crossed, this is a sign of poisoning.